Ryegrass No. 1, 2, 3 weeds in Arkansas wheat

Many Mid-South farmers would confess to having some anxious moments when rain began falling in mid to late October, potentially disrupting plans for increasing their acres of winter wheat.

With soft red wheat futures trading at the unheard-of price of $8-plus per bushel for delivery next summer, growers wanted to get in all the acres they could before winter rains put an end to planting.

“We got about 100,000 acres of wheat planted in early October, and it began raining,” said Bob Scott, Extension weed specialist with the University of Arkansas. “It finally quit, and we’ve been planting every day since then.”

Speaking at the Delta Crop Summit at the Delta Research and Extension Center in Stoneville, Miss., on Nov. 13, Scott said Arkansas farmers could be pushing 1 million acres of winter wheat when it’s all said and done. “We haven’t been there in several years,” he noted.

Arkansas hasn’t planted more than a million acres of wheat since 2001. Last year’s acreage hit 820,000, and the year before, 365,000, according to USDA.

The increased acreage and high wheat prices mean controlling weeds will be of more than passing interest this winter, says Scott.

Although farmers can no longer apply a herbicide pre-emergence except on extremely late plantings, they still have postemergence herbicide options for controlling what Scott says are the three main weeds in Arkansas wheat — ryegrass, ryegrass and ryegrass.

One is pendimethalin or Prowl H2O, which is normally thought of as a pre-emergence or residual herbicide. Prowl, labeled for early postemergence application in wheat last year, can be applied at the one-leaf or two-leaf stages of wheat or tank-mixed with Sencor at the two-leaf stage.

Growers should avoid applying the tank mix or Sencor alone to metribuzin-sensitive varieties. Growers should talk to their seed dealers about the latter. Arkansas recommends the application of 2.1 pints of pendimethalin per acre on clay and mixed soils and 1.8 pints on sandy soils.

Pendimethalin can help prevent new flushes of ryegrass and also provides residual control of buttercup, shepherd’s purse and henbit, which Scott lists in a class of weeds he calls “winter annual junk.”

“Pendimethalin can also be a good tank mix partner with Osprey, Hoelon or Axial for control of ryegrass in the fall,” says Scott. “And it has another advantage of being relatively cheap.”

Growers should be cautious in applying Hoelon because Hoelon-resistant Italian ryegrass has been identified in every wheat-growing county in Arkansas and some Mid-South states. The first case of Hoelon-resistant ryegrass was documented in a field near Willow Beach, Ark.

Osprey, a relatively new sulfonylurea compound from Bayer CropScience, is a total postemergence herbicide that can be applied at the rate of 4.75 ounces per acre with a methylated seed oil at 1 percent or 0.5 percent non-ionic surfactant plus UAN.

Because it has no residual activity, farmers may delay application of Osprey to control late-emerging flushes of Italian and perennial ryegrass, the two primary species of the weed found in Arkansas and other Mid-South states.

Scott believes fall is the best time for controlling ryegrass. “My thinking is that you spray herbicides such as Osprey in the fall for yield and in the spring for convenience,” he said. “If you control ryegrass early, you have less time for it to compete with the wheat.

“Studies have shown that by the time it warms up enough in the spring to spray materials such as Osprey, it may be too late to prevent yield losses from ryegrass competition in wheat.” Temperatures below 45 degrees F can reduce the activity of Osprey.

Adding Prowl to a fall application of Osprey can also help knock back late-emerging ryegrass.

Growers who prefer to apply Osprey in the spring and try to control ryegrass with one pass can tank mix it with Harmony Extra for wild garlic. Osprey also controls annual bluegrass and has activity on broadleaf weeds except mayweed and wild onion or garlic.

“If a grower hasn’t been using Hoelon, it is still an excellent option for control of emerged ryegrass in both fall and spring,” said Scott. “The same can be said for Axial, which is from the same family of chemicals and has the same mode of action as Hoelon.”

The primary problem with applying Hoelon in the spring may be the expense. Most weed scientists recommend an application of 1.33 pints of Hoelon in the fall and up to 2.67 pints in the spring, depending on the ryegrass size and number of tillers.

“Compared to a pre-emergence rate of a product such as Finesse at $6 per acre, an application of Hoelon can run over $20 per acre,” says Scott.

Axial, a new herbicide from Syngenta, is a slightly different acc-ase inhibitor, but is a member of the same chemical family as Hoelon, according to Scott. The recommended rate is 8.2 ounces of product plus 9.6 ounces of Adigor adjuvant unless it is included in the formulation.

In fields with sizable populations of ryegrass, growers may need to try a multi-step approach, says Scott. That could include fallowing a field for a year or allowing ryegrass to emerge before disking or burning down the field. Applying Finesse pre-emergence in the fall followed by Osprey, Hoelon or Axial in the fall and Osprey in the spring may also help.

Growers should also be on the lookout for horseweed or marestail in wheat. Horseweed typically is not a big competitor for nutrients in wheat, but Scott recommends growers go after it if they spot it in their fields.

“I have a new credo that you should never pass up a chance to kill a horseweed,” he told farmers attending the Crop Summit, which was organized by researchers and Extension specialists at the DREC.

“Most of the horseweed in Arkansas is resistant to glyphosate. Finesse or Prowl applied pre-emergence will control it, but my favorite treatment is 0.5 ounce of Harmony Extra plus 2,4-D. I would consider adding 4 ounces of dicamba or Clarity for very severe infestations.” (The latter may cause injury to the wheat and should not be applied past joint movement, he notes.)

Scott says farmers have few options for controlling glyphosate-resistant horseweed after wheat is harvested or after soybeans are planted in the spring. “You can cut them off with the combine and have three plants where you had one,” he said. “That’s why I think it is better to kill them sooner rather than later.”

Scott says even light infestations of ryegrass can have a negative impact on yield. In general, broadleaf weeds have less impact but can cause harvest difficulties and impair grain quality if left unchecked.

“Only rarely will weeds such as, henbit, mayweed, mustards and garlic and onion actually cause yield losses from weed competition,” he said. “Usually there has to be some set of circumstances, such as, a thin stand of wheat, late wheat, drought, or weeds present at planting for these weeds to hurt wheat yields.”